This invention relates to a multistage hydraulic machine, and more particularly, a structure of a turbine shaft of a multistage pump-turbine for improving rigidity of the turbine shaft against bending stress.
A multistage pump-turbine is generally used when a head in a hydraulic power station increases and is over limitation of operation performance of a single-stage pump-turbine. From this point of view, especially, it has highly been required to provide a two-stage pump-turbine in which movable guide vanes and their control means are relatively easily assembled at the peripheral portions of runners of the respective pressure stages in comparison with the other multistage pump-turbines having more than two pressure stages.
With multistage pump-turbines of the type described above, the fixing of a plurality of runners on one turbine shaft includes troublesome assembling operations in comparison with a case where a single-stage pump-turbine is used, and when a multistage pump-turbine with small capacity is used, a plurality of runners are shrink-fitted on one turbine shaft to thereby transfer torque and support a bending moment caused by a radially horizontal unbalancing force applied on the runners. In a multistage pump-turbine with large capacity, however, the turbine shaft inevitably becomes heavy and elongated, which complicates operations such as transportation and assembling or disassembling operations.
In order to obviate these troublesome operations, there has been proposed a turbine shaft which is dividable into two or three parts, for example, in transportation, and assembled in actual field work.
Actually, in a conventional two-stage pump-turbine having a turbine shaft which consists of upper and lower portions, the runner of the high pressure stage is fixed by bolt means to the lower end of the upper shaft and the runner of the low pressure stage is fixed by bolt means to the lower shaft. The upper portion of the lower shaft is fitted into a connection hole formed at a runner cone of the high pressure stage and fixed by key members and bolt and nut means. In the other conventional shaft structure having upper and lower shaft portions and an intermediate shaft portion disposed therebetween, the runner of the low pressure stage is fixed by bolt means to a flanged portion of the intermediate shaft and also is fixed to the lower shaft by bolt means.
Since the multistage pump-turbine thus constructed is provided with a considerably long turbine shaft in comparison with that of a single-stage pump-turbine, there arises a problem in lowering a critical speed of the shaft system that shows the degree of stability against vibration force caused by the runners during operation. Particularly, with the highest pressure stage operated under the highest pressure and provided with a spiral casing having an asymmetric water passage with respect to the turbine shaft, a large hydraulic disequilibrium force is inevitably applied to the highest pressure stage in comparison with the other pressure stages, thus requiring the highest bending rigidity for the turbine shaft structure at the highest pressure stage. From this viewpoint, the turbine shaft structure, i.e. the connecting portions of the upper shaft part to the lower and intermediate shaft parts, of the conventional multistage pump-turbine cannot provide a sufficient bending rigidity against the hydraulic disequilibrium force.